Carbon-free phosphoric acid can be used to make carbon-free ammonium polyphosphate solution by reaction of ammonia with the acid. However, carbon-free phosphoric acid requires that the phosphate rock from which the acid is produced be calcined to burn off organic residues. The energy required for calcining the rock can be saved if the calcination can be avoided; however, the acid produced from uncalcined rock contains carbonaceous residues. Further, ammonium polyphosphate solutions made from this "black acid" still contain these carbonaceous impurities especially if made by the direct ammoniation process which increases the amount of carbonaceous contaminant in the end product.
In the ammonium polyphosphate solution, made from carbon-contaminated phosphoric acid, it is common to find fine white particles which are formed from phosphates of metallic impurities like aluminum, iron and calcium in the system. These white impurities are commonly in suspension so that the ammonium polyphosphate can be said to be a solution/slurry. Since the white particulate contains phosphate, it has fertilizer values and should be retained in suspension. Thus the problem is one of removing black carbonaceous materials without removing the useful white phosphate-containing particulate, and the normal methods of solid/liquid separation, e.g., filtration, cannot be used. Many efforts have been made to clarify these carbon contaminated solutions without removal of the phosphate solids which indicates the seriousness of the problem.
Thus, in U.S. Pat. No. 3,619,161 (Nov. 9, 1971) a process has been described wherein an organic stratifying agent, such as alkyl benzene sulfonates, tall oil, tall oil fatty acids and ethoxylated fatty alcohols and acids, are added to the impure solutions. The treatment results in the generation of an organic phase containing the stratifying agent and a concentration of carbonaceous impurities and a polyphosphate phase of reduced carbonaceous impurity content. Although the treatment removes some carbonaceous impurities from the polyphosphate phase, the residual black color was found to be still objectionable.
In U.S. Pat. No. 3,630,711 (Dec. 28, 1971) it has been suggested that the same carbonaceous impurities can be removed from ammonium polyphosphate solutions by treating the solution with an aliphatic organic amine compound, such as having 12-22 carbon atoms. The addition of amines to the impure ammonium polyphosphate solution results in improved clarity, however, the color improvement is still below the desired limits and the solution treated with the organic amine often fails to pass the required color standards.
To improve the deficiencies of the prior art purification processes it has been recommended in U.S. Pat. No. 3,969,483 (July 13, 1976) to employ as carbonaceous impurity-removing agent a mixture of a water-insoluble organic primary amine and a quaternary ammonium chloride. The recommended mixture provides improved clarity to the ammonium polyphosphate solution, but in many instances the treatment still results in an insufficient degree of carbonaceous impurity removal and the treated solution will be commercially unacceptable.
It has been now found that significantly improved carbonaceous impurity removal can be achieved from contaminated ammonium polyphosphate solutions by a sequential treatment with a combination of treating agents. The significantly improved carbonaceous impurity removal can be achieved by first adding to the impure solution a mixture of a water-insoluble, organic primary amine and a quaternary ammonium salt, followed by the addition of an oil, for example, fuel oil. The sequential treatment results in excellent color improvement, which cannot be achieved either by the prior art methods referred to above or by reversing the sequence of addition of the treating agents.